<head>
<title>Exception Handling in LLVM</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
- <meta name="description"
+ <meta name="description"
content="Exception Handling in LLVM.">
- <link rel="stylesheet" href="llvm.css" type="text/css">
+ <link rel="stylesheet" href="_static/llvm.css" type="text/css">
</head>
<body>
-<div class="doc_title">Exception Handling in LLVM</div>
+<h1>Exception Handling in LLVM</h1>
<table class="layout" style="width:100%">
<tr class="layout">
<li><a href="#introduction">Introduction</a>
<ol>
<li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li>
+ <li><a href="#sjlj">Setjmp/Longjmp Exception Handling</a></li>
<li><a href="#overview">Overview</a></li>
</ol></li>
<li><a href="#codegen">LLVM Code Generation</a>
</ol></li>
<li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
<ol>
- <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a></li>
- <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a></li>
<li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>
<li><a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a></li>
<li><a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a></li>
<li><a href="#unwind_tables">Exception Handling Frame</a></li>
<li><a href="#exception_tables">Exception Tables</a></li>
</ol></li>
- <li><a href="#todo">ToDo</a></li>
</ul>
</td>
</tr></table>
<div class="doc_author">
- <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
+ <p>Written by the <a href="http://llvm.org/">LLVM Team</a></p>
</div>
<!-- *********************************************************************** -->
-<div class="doc_section"><a name="introduction">Introduction</a></div>
+<h2><a name="introduction">Introduction</a></h2>
<!-- *********************************************************************** -->
-<div class="doc_text">
+<div>
<p>This document is the central repository for all information pertaining to
exception handling in LLVM. It describes the format that LLVM exception
handling information takes, which is useful for those interested in creating
front-ends or dealing directly with the information. Further, this document
provides specific examples of what exception handling information is used for
- in C/C++.</p>
-
-</div>
+ in C and C++.</p>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>Exception handling for most programming languages is designed to recover from
conditions that rarely occur during general use of an application. To that
Exception Handling</a>. A description of the exception frame format can be
found at
<a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception
- Frames</a>, with details of the DWARF 3 specification at
- <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3 Standard</a>.
+ Frames</a>, with details of the DWARF 4 specification at
+ <a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 Standard</a>.
A description for the C++ exception table formats can be found at
<a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
Tables</a>.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="overview">Overview</a>
+<h3>
+ <a name="sjlj">Setjmp/Longjmp Exception Handling</a>
+</h3>
+
+<div>
+
+<p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
+ <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and
+ <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to
+ handle control flow for exception handling.</p>
+
+<p>For each function which does exception processing — be
+ it <tt>try</tt>/<tt>catch</tt> blocks or cleanups — that function
+ registers itself on a global frame list. When exceptions are unwinding, the
+ runtime uses this list to identify which functions need processing.<p>
+
+<p>Landing pad selection is encoded in the call site entry of the function
+ context. The runtime returns to the function via
+ <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where
+ a switch table transfers control to the appropriate landing pad based on
+ the index stored in the function context.</p>
+
+<p>In contrast to DWARF exception handling, which encodes exception regions
+ and frame information in out-of-line tables, SJLJ exception handling
+ builds and removes the unwind frame context at runtime. This results in
+ faster exception handling at the expense of slower execution when no
+ exceptions are thrown. As exceptions are, by their nature, intended for
+ uncommon code paths, DWARF exception handling is generally preferred to
+ SJLJ.</p>
+
</div>
-<div class="doc_text">
+<!-- ======================================================================= -->
+<h3>
+ <a name="overview">Overview</a>
+</h3>
+
+<div>
<p>When an exception is thrown in LLVM code, the runtime does its best to find a
handler suited to processing the circumstance.</p>
<p>The runtime first attempts to find an <i>exception frame</i> corresponding to
the function where the exception was thrown. If the programming language
- (e.g. C++) supports exception handling, the exception frame contains a
+ supports exception handling (e.g. C++), the exception frame contains a
reference to an exception table describing how to process the exception. If
- the language (e.g. C) does not support exception handling, or if the
+ the language does not support exception handling (e.g. C), or if the
exception needs to be forwarded to a prior activation, the exception frame
contains information about how to unwind the current activation and restore
the state of the prior activation. This process is repeated until the
- exception is handled. If the exception is not handled and no activations
+ exception is handled. If the exception is not handled and no activations
remain, then the application is terminated with an appropriate error
message.</p>
<p>Because different programming languages have different behaviors when
handling exceptions, the exception handling ABI provides a mechanism for
- supplying <i>personalities.</i> An exception handling personality is defined
+ supplying <i>personalities</i>. An exception handling personality is defined
by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt>
in C++), which receives the context of the exception, an <i>exception
structure</i> containing the exception object type and value, and a reference
for the current compile unit is specified in a <i>common exception
frame</i>.</p>
-<p>The organization of an exception table is language dependent. For C++, an
+<p>The organization of an exception table is language dependent. For C++, an
exception table is organized as a series of code ranges defining what to do
- if an exception occurs in that range. Typically, the information associated
+ if an exception occurs in that range. Typically, the information associated
with a range defines which types of exception objects (using C++ <i>type
info</i>) that are handled in that range, and an associated action that
- should take place. Actions typically pass control to a <i>landing
+ should take place. Actions typically pass control to a <i>landing
pad</i>.</p>
-<p>A landing pad corresponds to the code found in the <i>catch</i> portion of
- a <i>try</i>/<i>catch</i> sequence. When execution resumes at a landing
- pad, it receives the exception structure and a selector corresponding to
- the <i>type</i> of exception thrown. The selector is then used to determine
- which <i>catch</i> should actually process the exception.</p>
+<p>A landing pad corresponds roughly to the code found in the <tt>catch</tt>
+ portion of a <tt>try</tt>/<tt>catch</tt> sequence. When execution resumes at
+ a landing pad, it receives an <i>exception structure</i> and a
+ <i>selector value</i> corresponding to the <i>type</i> of exception
+ thrown. The selector is then used to determine which <i>catch</i> should
+ actually process the exception.</p>
</div>
-<!-- ======================================================================= -->
-<div class="doc_section">
- <a name="codegen">LLVM Code Generation</a>
</div>
-<div class="doc_text">
+<!-- ======================================================================= -->
+<h2>
+ <a name="codegen">LLVM Code Generation</a>
+</h2>
-<p>At the time of this writing, only C++ exception handling support is available
- in LLVM. So the remainder of this document will be somewhat C++-centric.</p>
+<div>
-<p>From the C++ developers perspective, exceptions are defined in terms of the
- <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section
+<p>From a C++ developer's perspective, exceptions are defined in terms of the
+ <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section
we will describe the implementation of LLVM exception handling in terms of
C++ examples.</p>
-</div>
-
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="throw">Throw</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>Languages that support exception handling typically provide a <tt>throw</tt>
- operation to initiate the exception process. Internally, a throw operation
- breaks down into two steps. First, a request is made to allocate exception
- space for an exception structure. This structure needs to survive beyond the
- current activation. This structure will contain the type and value of the
- object being thrown. Second, a call is made to the runtime to raise the
- exception, passing the exception structure as an argument.</p>
-
-<p>In C++, the allocation of the exception structure is done by
- the <tt>__cxa_allocate_exception</tt> runtime function. The exception
- raising is handled by <tt>__cxa_throw</tt>. The type of the exception is
- represented using a C++ RTTI structure.</p>
+ operation to initiate the exception process. Internally, a <tt>throw</tt>
+ operation breaks down into two steps.</p>
+
+<ol>
+ <li>A request is made to allocate exception space for an exception structure.
+ This structure needs to survive beyond the current activation. This
+ structure will contain the type and value of the object being thrown.</li>
+
+ <li>A call is made to the runtime to raise the exception, passing the
+ exception structure as an argument.</li>
+</ol>
+
+<p>In C++, the allocation of the exception structure is done by the
+ <tt>__cxa_allocate_exception</tt> runtime function. The exception raising is
+ handled by <tt>__cxa_throw</tt>. The type of the exception is represented
+ using a C++ RTTI structure.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="try_catch">Try/Catch</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>A call within the scope of a <i>try</i> statement can potentially raise an
- exception. In those circumstances, the LLVM C++ front-end replaces the call
- with an <tt>invoke</tt> instruction. Unlike a call, the <tt>invoke</tt> has
- two potential continuation points: where to continue when the call succeeds
- as per normal; and where to continue if the call raises an exception, either
- by a throw or the unwinding of a throw.</p>
+ exception. In those circumstances, the LLVM C++ front-end replaces the call
+ with an <tt>invoke</tt> instruction. Unlike a call, the <tt>invoke</tt> has
+ two potential continuation points:</p>
+
+<ol>
+ <li>where to continue when the call succeeds as per normal, and</li>
+
+ <li>where to continue if the call raises an exception, either by a throw or
+ the unwinding of a throw</li>
+</ol>
<p>The term used to define a the place where an <tt>invoke</tt> continues after
- an exception is called a <i>landing pad</i>. LLVM landing pads are
+ an exception is called a <i>landing pad</i>. LLVM landing pads are
conceptually alternative function entry points where an exception structure
- reference and a type info index are passed in as arguments. The landing pad
+ reference and a type info index are passed in as arguments. The landing pad
saves the exception structure reference and then proceeds to select the catch
block that corresponds to the type info of the exception object.</p>
-<p>Two LLVM intrinsic functions are used to convey information about the landing
- pad to the back end.</p>
-
-<ol>
- <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no
- arguments and returns a pointer to the exception structure. This only
- returns a sensible value if called after an <tt>invoke</tt> has branched
- to a landing pad. Due to code generation limitations, it must currently
- be called in the landing pad itself.</li>
-
- <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum
- of three arguments. The first argument is the reference to the exception
- structure. The second argument is a reference to the personality function
- to be used for this <tt>try</tt>/<tt>catch</tt> sequence. Each of the
- remaining arguments is either a reference to the type info for
- a <tt>catch</tt> statement, a <a href="#throw_filters">filter</a>
- expression, or the number zero (<tt>0</tt>) representing
- a <a href="#cleanups">cleanup</a>. The exception is tested against the
- arguments sequentially from first to last. The result of
- the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
- positive number if the exception matched a type info, a negative number if
- it matched a filter, and zero if it matched a cleanup. If nothing is
- matched, the behaviour of the program
- is <a href="#restrictions">undefined</a>. This only returns a sensible
- value if called after an <tt>invoke</tt> has branched to a landing pad.
- Due to codegen limitations, it must currently be called in the landing pad
- itself. If a type info matched, then the selector value is the index of
- the type info in the exception table, which can be obtained using the
- <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a>
- intrinsic.</li>
-</ol>
+<p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
+ instruction</a> is used to convey information about the landing pad to the
+ back end. For C++, the <tt>landingpad</tt> instruction returns a pointer and
+ integer pair corresponding to the pointer to the <i>exception structure</i>
+ and the <i>selector value</i> respectively.</p>
+
+<p>The <tt>landingpad</tt> instruction takes a reference to the personality
+ function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The
+ remainder of the instruction is a list of <i>cleanup</i>, <i>catch</i>,
+ and <i>filter</i> clauses. The exception is tested against the clauses
+ sequentially from first to last. The selector value is a positive number if
+ the exception matched a type info, a negative number if it matched a filter,
+ and zero if it matched a cleanup. If nothing is matched, the behavior of
+ the program is <a href="#restrictions">undefined</a>. If a type info matched,
+ then the selector value is the index of the type info in the exception table,
+ which can be obtained using the
+ <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
<p>Once the landing pad has the type info selector, the code branches to the
- code for the first catch. The catch then checks the value of the type info
+ code for the first catch. The catch then checks the value of the type info
selector against the index of type info for that catch. Since the type info
- index is not known until all the type info have been gathered in the backend,
- the catch code will call the
- <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic
- to determine the index for a given type info. If the catch fails to match
- the selector then control is passed on to the next catch. Note: Since the
- landing pad will not be used if there is no match in the list of type info on
- the call to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then
- neither the last catch nor <i>catch all</i> need to perform the check
- against the selector.</p>
-
-<p>Finally, the entry and exit of catch code is bracketed with calls
- to <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
+ index is not known until all the type infos have been gathered in the
+ backend, the catch code must call the
+ <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
+ determine the index for a given type info. If the catch fails to match the
+ selector then control is passed on to the next catch.</p>
+
+<p>Finally, the entry and exit of catch code is bracketed with calls to
+ <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
<ul>
- <li><tt>__cxa_begin_catch</tt> takes a exception structure reference as an
+ <li><tt>__cxa_begin_catch</tt> takes an exception structure reference as an
argument and returns the value of the exception object.</li>
- <li><tt>__cxa_end_catch</tt> takes a exception structure reference as an
- argument. This function clears the exception from the exception space.
- Note: a rethrow from within the catch may replace this call with
- a <tt>__cxa_rethrow</tt>.</li>
+ <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
+ <ol>
+ <li>Locates the most recently caught exception and decrements its handler
+ count,</li>
+ <li>Removes the exception from the <i>caught</i> stack if the handler
+ count goes to zero, and</li>
+ <li>Destroys the exception if the handler count goes to zero and the
+ exception was not re-thrown by throw.</li>
+ </ol>
+ <p><b>Note:</b> a rethrow from within the catch may replace this call with
+ a <tt>__cxa_rethrow</tt>.</p></li>
</ul>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="cleanups">Cleanups</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
-<p>To handle destructors and cleanups in <tt>try</tt> code, control may not run
- directly from a landing pad to the first catch. Control may actually flow
- from the landing pad to clean up code and then to the first catch. Since the
- required clean up for each <tt>invoke</tt> in a <tt>try</tt> may be different
- (e.g. intervening constructor), there may be several landing pads for a given
- try. If cleanups need to be run, the number zero should be passed as the
- last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
- However for C++ a <tt>null i8*</tt> <b><a href="#restrictions">must</a></b>
- be passed instead.</p>
+<p>A cleanup is extra code which needs to be run as part of unwinding a scope.
+ C++ destructors are a typical example, but other languages and language
+ extensions provide a variety of different kinds of cleanups. In general, a
+ landing pad may need to run arbitrary amounts of cleanup code before actually
+ entering a catch block. To indicate the presence of cleanups, a
+ <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
+ should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at
+ the landing pad if there are no catches or filters that require it to.</p>
-</div>
-
-<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="throw_filters">Throw Filters</a>
-</div>
-
-<div class="doc_text">
-
-<p>C++ allows the specification of which exception types can be thrown from a
- function. To represent this a top level landing pad may exist to filter out
- invalid types. To express this in LLVM code the landing pad will
- call <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The
- arguments are a reference to the exception structure, a reference to the
- personality function, the length of the filter expression (the number of type
- infos plus one), followed by the type infos themselves.
- <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a
- negative value if the exception does not match any of the type infos. If no
- match is found then a call to <tt>__cxa_call_unexpected</tt> should be made,
- otherwise <tt>_Unwind_Resume</tt>. Each of these functions requires a
- reference to the exception structure. Note that the most general form of an
- <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call can contain
- any number of type infos, filter expressions and cleanups (though having more
- than one cleanup is pointless). The LLVM C++ front-end can generate such
- <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls due to
- inlining creating nested exception handling scopes.</p>
-
-</div>
-
-<!-- ======================================================================= -->
-<div class="doc_subsection">
- <a name="restrictions">Restrictions</a>
-</div>
+<p><b>Note:</b> Do not allow a new exception to propagate out of the execution
+ of a cleanup. This can corrupt the internal state of the unwinder.
+ Different languages describe different high-level semantics for these
+ situations: for example, C++ requires that the process be terminated, whereas
+ Ada cancels both exceptions and throws a third.</p>
-<div class="doc_text">
-
-<p>The semantics of the invoke instruction require that any exception that
- unwinds through an invoke call should result in a branch to the invoke's
- unwind label. However such a branch will only happen if the
- <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> matches. Thus in
- order to ensure correct operation, the front-end must only generate
- <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls that are
- guaranteed to always match whatever exception unwinds through the invoke.
- For most languages it is enough to pass zero, indicating the presence of
- a <a href="#cleanups">cleanup</a>, as the
- last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
- However for C++ this is not sufficient, because the C++ personality function
- will terminate the program if it detects that unwinding the exception only
- results in matches with cleanups. For C++ a <tt>null i8*</tt> should be
- passed as the last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>
- argument instead. This is interpreted as a catch-all by the C++ personality
- function, and will always match.</p>
+<p>When all cleanups are finished, if the exception is not handled by the
+ current function, resume unwinding by calling the
+ <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in
+ the result of the <tt>landingpad</tt> instruction for the original landing
+ pad.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_section">
- <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
-</div>
+<h3>
+ <a name="throw_filters">Throw Filters</a>
+</h3>
-<div class="doc_text">
+<div>
-<p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to
- provide exception handling information at various points in generated
- code.</p>
+<p>C++ allows the specification of which exception types may be thrown from a
+ function. To represent this, a top level landing pad may exist to filter out
+ invalid types. To express this in LLVM code the
+ <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will
+ have a filter clause. The clause consists of an array of type infos.
+ <tt>landingpad</tt> will return a negative value if the exception does not
+ match any of the type infos. If no match is found then a call
+ to <tt>__cxa_call_unexpected</tt> should be made, otherwise
+ <tt>_Unwind_Resume</tt>. Each of these functions requires a reference to the
+ exception structure. Note that the most general form of a
+ <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can
+ have any number of catch, cleanup, and filter clauses (though having more
+ than one cleanup is pointless). The LLVM C++ front-end can generate such
+ <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due
+ to inlining creating nested exception handling scopes.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsubsection">
- <a name="llvm_eh_exception">llvm.eh.exception</a>
-</div>
+<h3>
+ <a name="restrictions">Restrictions</a>
+</h3>
-<div class="doc_text">
+<div>
-<pre>
- i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>( )
-</pre>
+<p>The unwinder delegates the decision of whether to stop in a call frame to
+ that call frame's language-specific personality function. Not all unwinders
+ guarantee that they will stop to perform cleanups. For example, the GNU C++
+ unwinder doesn't do so unless the exception is actually caught somewhere
+ further up the stack.</p>
-<p>This intrinsic returns a pointer to the exception structure.</p>
+<p>In order for inlining to behave correctly, landing pads must be prepared to
+ handle selector results that they did not originally advertise. Suppose that
+ a function catches exceptions of type <tt>A</tt>, and it's inlined into a
+ function that catches exceptions of type <tt>B</tt>. The inliner will update
+ the <tt>landingpad</tt> instruction for the inlined landing pad to include
+ the fact that <tt>B</tt> is also caught. If that landing pad assumes that it
+ will only be entered to catch an <tt>A</tt>, it's in for a rude awakening.
+ Consequently, landing pads must test for the selector results they understand
+ and then resume exception propagation with the
+ <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of
+ the conditions match.</p>
</div>
-<!-- ======================================================================= -->
-<div class="doc_subsubsection">
- <a name="llvm_eh_selector">llvm.eh.selector</a>
</div>
-<div class="doc_text">
-
-<pre>
- i32 %<a href="#llvm_eh_selector">llvm.eh.selector.i32</a>(i8*, i8*, i8*, ...)
- i64 %<a href="#llvm_eh_selector">llvm.eh.selector.i64</a>(i8*, i8*, i8*, ...)
-</pre>
+<!-- ======================================================================= -->
+<h2>
+ <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
+</h2>
-<p>This intrinsic is used to compare the exception with the given type infos,
- filters and cleanups.</p>
-
-<p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
- three arguments. The first argument is the reference to the exception
- structure. The second argument is a reference to the personality function to
- be used for this try catch sequence. Each of the remaining arguments is
- either a reference to the type info for a catch statement,
- a <a href="#throw_filters">filter</a> expression, or the number zero
- representing a <a href="#cleanups">cleanup</a>. The exception is tested
- against the arguments sequentially from first to last. The result of
- the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a positive
- number if the exception matched a type info, a negative number if it matched
- a filter, and zero if it matched a cleanup. If nothing is matched, the
- behaviour of the program is <a href="#restrictions">undefined</a>. If a type
- info matched then the selector value is the index of the type info in the
- exception table, which can be obtained using the
- <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
+<div>
-</div>
+<p>In addition to the
+ <a href="LangRef.html#i_landingpad"><tt>landingpad</tt></a> and
+ <a href="LangRef.html#i_resume"><tt>resume</tt></a> instructions, LLVM uses
+ several intrinsic functions (name prefixed with <i><tt>llvm.eh</tt></i>) to
+ provide exception handling information at various points in generated
+ code.</p>
<!-- ======================================================================= -->
-<div class="doc_subsubsection">
+<h4>
<a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<pre>
- i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for.i32</a>(i8*)
- i64 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for.i64</a>(i8*)
+ i32 @llvm.eh.typeid.for(i8* %type_info)
</pre>
<p>This intrinsic returns the type info index in the exception table of the
current function. This value can be used to compare against the result
- of <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The single
- argument is a reference to a type info.</p>
+ of <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>.
+ The single argument is a reference to a type info.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsubsection">
+<h4>
<a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<pre>
- i32 %<a href="#llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>(i8*)
+ i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)
</pre>
-<p>The SJLJ exception handling uses this intrinsic to force register saving for
- the current function and to store the address of the following instruction
- for use as a destination address by <a href="#llvm_eh_sjlj_longjmp">
- <tt>llvm.eh.sjlj.longjmp</tt></a>. The buffer format and the overall
- functioning of this intrinsic is compatible with the GCC
- <tt>__builtin_setjmp</tt> implementation, allowing code built with the
- two compilers to interoperate.</p>
+<p>For SJLJ based exception handling, this intrinsic forces register saving for
+ the current function and stores the address of the following instruction for
+ use as a destination address
+ by <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>. The
+ buffer format and the overall functioning of this intrinsic is compatible
+ with the GCC <tt>__builtin_setjmp</tt> implementation allowing code built
+ with the clang and GCC to interoperate.</p>
<p>The single parameter is a pointer to a five word buffer in which the calling
context is saved. The front end places the frame pointer in the first word,
</div>
<!-- ======================================================================= -->
-<div class="doc_subsubsection">
- <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
+<h4>
+ <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
+</h4>
+
+<div>
+
+<pre>
+ void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)
+</pre>
+
+<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.longjmp</tt>
+ intrinsic is used to implement <tt>__builtin_longjmp()</tt>. The single
+ parameter is a pointer to a buffer populated
+ by <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a>. The frame
+ pointer and stack pointer are restored from the buffer, then control is
+ transferred to the destination address.</p>
+
</div>
+<!-- ======================================================================= -->
+<h4>
+ <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
+</h4>
-<div class="doc_text">
+<div>
<pre>
- i8* %<a href="#llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>( )
+ i8* @llvm.eh.sjlj.lsda()
</pre>
-<p>Used for SJLJ based exception handling, the <a href="#llvm_eh_sjlj_lsda">
- <tt>llvm.eh.sjlj.lsda</tt></a> intrinsic returns the address of the Language
- Specific Data Area (LSDA) for the current function. The SJLJ front-end code
- stores this address in the exception handling function context for use by the
- runtime.</p>
+<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.lsda</tt> intrinsic
+ returns the address of the Language Specific Data Area (LSDA) for the current
+ function. The SJLJ front-end code stores this address in the exception
+ handling function context for use by the runtime.</p>
</div>
<!-- ======================================================================= -->
-<div class="doc_subsubsection">
+<h4>
<a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
-</div>
+</h4>
-<div class="doc_text">
+<div>
<pre>
- void %<a href="#llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>(i32)
+ void @llvm.eh.sjlj.callsite(i32 %call_site_num)
</pre>
-<p>The SJLJ front-end allocates call site indices for invoke instrucitons.
- These values are passed to the back-end via the
- <a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a>
- intrinsic, where they are used to build the LSDA call-site table.</p>
+<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.callsite</tt>
+ intrinsic identifies the callsite value associated with the
+ following <tt>invoke</tt> instruction. This is used to ensure that landing
+ pad entries in the LSDA are generated in matching order.</p>
+
+</div>
</div>
<!-- ======================================================================= -->
-<div class="doc_section">
+<h2>
<a name="asm">Asm Table Formats</a>
-</div>
+</h2>
-<div class="doc_text">
+<div>
<p>There are two tables that are used by the exception handling runtime to
- determine which actions should take place when an exception is thrown.</p>
-
-</div>
+ determine which actions should be taken when an exception is thrown.</p>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="unwind_tables">Exception Handling Frame</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
- frame used by dwarf debug info. The frame contains all the information
+ frame used by DWARF debug info. The frame contains all the information
necessary to tear down the current frame and restore the state of the prior
- frame. There is an exception handling frame for each function in a compile
+ frame. There is an exception handling frame for each function in a compile
unit, plus a common exception handling frame that defines information common
to all functions in the unit.</p>
-<p>Todo - Table details here.</p>
+<!-- Todo - Table details here. -->
</div>
<!-- ======================================================================= -->
-<div class="doc_subsection">
+<h3>
<a name="exception_tables">Exception Tables</a>
-</div>
+</h3>
-<div class="doc_text">
+<div>
<p>An exception table contains information about what actions to take when an
- exception is thrown in a particular part of a function's code. There is one
- exception table per function except leaf routines and functions that have
- only calls to non-throwing functions will not need an exception table.</p>
-
-<p>Todo - Table details here.</p>
+ exception is thrown in a particular part of a function's code. There is one
+ exception table per function, except leaf functions and functions that have
+ calls only to non-throwing functions. They do not need an exception
+ table.</p>
-</div>
+<!-- Todo - Table details here. -->
-<!-- ======================================================================= -->
-<div class="doc_section">
- <a name="todo">ToDo</a>
</div>
-<div class="doc_text">
-
-<ol>
-
- <li>Testing/Testing/Testing.</li>
-
-</ol>
-
</div>
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- <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
- <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
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